The invention relates to an automated process for the antistatic coating of plastic moulding surfaces or for the antistatic coating of plastic-based or lacquer-based coatings.
Plastic mouldings or plastic-based and/or lacquer-based coatings are currently used in many ways. They are frequently used e.g. as plastic add-on parts or plastic-coated/lacquer-coated parts in the automotive industry and other branches of industry, and in plastics manufacturing sectors. The antistatic behaviour has many problems associated with it, especially where plastic surfaces are machined e.g. by grinding.
The plastic mouldings can be produced in any desired manner. Their manufacturing processes, their structure and their composition are known in principle. The plastic mouldings are preferably produced by injection moulding or by casting into a mould, optionally with the use of a reinforcing matrix.
The production of plastic coatings, e.g. on construction elements such as panelling, on furniture or as films, or the production of lacquer coatings, is also known in principle. In terms of the present patent application, “lacquer” also includes lacquer-like polymer coatings. Mouldings that are only partially lacquered, for which adjacent plastic and lacquer have to be protected simultaneously from high electrical charge, are e.g. partially lacquered mouldings or add-on parts.
Because of their susceptibility to electrostatic charge, plastic mouldings or plastic and/or lacquer coatings are at particular risk of contamination and often need antistatic protection.
Nowadays, plastic mouldings conventionally have to be washed prior to lacquering so as to remove as much contamination as possible from the surfaces to be lacquered. This process is intended to remove, inter alia, the contamination acquired during transportation and/or storage.
An example of a washing process suitable for this purpose is the powerwash process, in which the mouldings are treated with water, e.g. by spraying or dipping. It can be carried out at a pressure ranging from 0.5 to 2 bar without the mouldings being clamped or supported, usually at a pressure of up to 2 bar in the case of mouldings in a fixed position, or optionally at a higher pressure. The wash waters used can have a pH preferably ranging from 2 to 12 as they do not usually attack the plastic and lacquer surfaces. Mouldings coated with plastic and/or lacquer can consist substantially of metal, glass, ceramic, wood, plastic, composite materials and/or material composites, etc. The plastic coating can be applied in one or more layers. This pressure washing process is described on page 9 of the GE Plastics “Painting guide”, edition August 1998.
An acidic wash water for such a washing process often contains at least one inorganic and/or organic acid, for example phosphoric acid, gluconic acid and/or glycolic acid. The wash water can also contain at least one corrosion inhibitor. The acid can be buffered and/or partially neutralized.
An alkaline wash water often contains at least one alkalinity carrier selected from the group comprising e.g. a carbonate, a hydroxide, an alkaline salt and an amine such as an alkanolamine.
The wash water usually also contains at least one surfactant and optionally further additives as well, e.g. at least one sequestering agent for dispersing the suspended matter, biocides and/or solubilizers.
If such a wash water is used at a pressure of up to 2 bar, or even more, for the treatment of a contaminated moulding surface, only the loose dirt not adhering electrostatically is removed. The electrostatically adhering dirt, which often consists substantially of specifically charged particles having sizes ranging preferentially from 0.05 to 5 μm, can normally only be removed to a very limited extent by a washing process under pressure.
Automated washing processes other than the powerwash process which are otherwise frequently used prior to lacquering are normally unsuitable for the removal of contamination occurring as a result of antistatic charge.
At present, the only possible way of removing electrostatically adhering dirt is mechanical removal by wiping or brushing. The manual removal of electrostatically adhering dirt immediately before the powerwash process is often no longer justifiable in mass production. Solutions normally based on isopropanol and/or water may be used for this purpose.
However, the cost of the measures taken to avoid electrostatically adhering dirt, such as the dust-free packaging of parts in sealed containers or storage in dust-free rooms, is many times the normal cost of storing parts treated with antistatic agents. The cost of totally removing dust is extremely high because it demands air filtration, locks, dust-avoiding conveying and transportation technology, dust-free clothing, appropriate worker behaviour, many structural requirements for avoiding dust and keeping it away, such as seals and floor structure, etc.
It has been found that the preparations and chemical compounds described in the literature for preventing electrostatic charge arising due to friction, and preventing moulding surfaces from becoming laden with electrostatically adhering dirt, are unsuitable for the subsequent powerwash process. To the Applicant's knowledge, the only known solutions for the antistatic coating of plastic surfaces to avoid charging are those which contain quaternary ammonium compounds. However, these cannot be used on their own because of the very intense foaming in the subsequent powerwash process.
EP 0 971 005 A1 describes a process for the temporary protection of plastic objects from electrical charge and the associated contamination using a composition containing conductive polymer as antistatic agent and water and/or organic solvent.
DE 197 07 650 A1 relates to antistatic cleaning agent concentrates that produce an antistatic cleaning solution when water is added, said concentrates comprising alkoxylated primary or secondary amines, fatty acid salts, e.g. alkanolamines, and low-foaming non-ionic surfactants, optionally with other auxiliary substances and active ingredients.
GB 2 203 162 A protects aqueous antistatic solutions based on water-soluble non-ionic antistatic agents and coconut oil fatty acid amide, the latter being said to act as solubilizer to stabilize the composition. The Examples also mention that the composition contains a quaternary ammonium compound and a cosolvent.
U.S. Pat. No.3,044,962 A teaches a process for the preparation of a cleaning composition from 1. an alcoholic solution of a surface-active agent selected from quaternary ammonium, phosphonium and morpholine salts, 2. a sodium salt, and 3. an organic anionic surface-active agent.
EP 1 111 036 A2 relates to a cleaning medium containing a dispersible abrasive which is said to act as an acidic abrasive cleaner.
U.S. Pat. No. 4,371,489 A protects a process for the production of molecularly orientated thermoplastic films with an antistatic film-forming coating comprising an acrylic/methacrylic copolymer, a crosslinking agent and a partially neutralized, acidic phosphate ester of defined composition.
Finally, DE 195 26 742 A1 relates to a polymer dispersion containing a water-soluble or finely dispersed antistatic agent. The polymer dispersion contains 50 to 95 wt. % of polymer.
One object is therefore to propose a process in which the plastic moulding surface or the plastic and/or lacquer coating is coated in such a way that the surface is not more strongly electrostatically charged, so that the dirt is not more strongly electrostatically attracted due to the field of the plastic or lacquer and cannot collect in a more strongly electrostatically charged state. Another object is to overcome the disadvantages of the state of the art and, in particular, to propose a process for avoiding dirt on organic surfaces which is difficult to remove, which process can be carried out prior to storage in the simplest possible and most cost-effective way, especially following the manufacture of a moulding or the coating of a moulding with a layer of plastic or lacquer, and does not interfere with the subsequent treatment in the powerwash process.
It has been found that, with the process according to the invention, the electrostatically charged dirt, consisting predominantly of particles in the size range from 0.05 to 5 μm, only settles on the moulding surface to a very small extent and/or in larger, readily removable particles, and that the contamination on the moulding surface coated according to the invention can be washed off unexpectedly well, e.g. in the powerwash process, with a wash water containing surfactant.
Because of the applied layer of surfactant, the plastic moulding surfaces antistatically coated according to the invention have more strongly hydrophilic surface properties that prevent a strong electrostatic charging of the plastic moulding surface or the plastic or lacquer surface.
The object is achieved by the process according to the present invention, which relates to an automated process for the antistatic coating of plastic molding surfaces after production of the plastic molding or after subsequent optional working of the molding or finishing of the surface, and before cleaning of the plastic molding by an aqueous washing process, and/or before polarization by chemical and/or physical methods, or for the antistatic coating of the surface of a plastic-coated and/or lacquer-coated molding, optionally after surface finishing, and before cleaning and/or polarization, the molding then optionally being lacquered, characterized in that an antistatic coating is applied by bringing the plastic and/or lacquer surface into contact with a solution containing at least one surfactant and at least 20 wt. % of at least one organic solvent and optionally water, the volatile constituents extensively or completely evaporating and a film containing surfactant being dried on, in order extensively or completely to prevent particle contamination due to electrostatic charging until the molding is cleaned and/or polarized, and in order to produce extensively or completely clean plastic and/or lacquer surfaces after the cleaning and/or polarization. The plastic and/or lacquer surfaces treated by the process may be used in the automotive industry and automotive component supply industry, for railway vehicles, for the aeronautical and space industry, for electrical engineering, electronics, domestic appliances, panelling, cladding elements, housings and clocks, for the building industry, for the furniture industry, in apparatus and machine engineering, in plastics manufacturing sectors or in sectors where plastic and/or lacquer surfaces are machined e.g. by drilling, deburring, milling or grindng.
Substantial evaporation signifies here that the surfactant film binds at least one molecular layer of water from the solution and/or from the ambient atmosphere. It is desirable for the solvent mixture to contain water because water, especially an increased water content and a reduced content of organic solvents, is more favourable for reasons of industrial safety, environmental protection and the costs of solvents or exhaust air treatment.
Coating with an antistatic solution is of particular interest as a temporary protection for storage, transportation and/or further processing by machining, coating processes and cleaning, and/or particularly before the final lacquering of mouldings coated in this way, especially where motor vehicle add-on parts are concerned. In such cases the solvents in the antistatic solution evaporate extensively. After drying, the solution produces a film whose weight often ranges from 0.5 to 500 mg/m2, preferably from 0.6 to 200 mg/m2, particularly preferably from 0.8 to 100 mg/m2 and very particularly preferably from 2 to 50 mg/m2.
In some cases a polarization of the plastic or the plastic or lacquer surface is carried out in order to produce polar groups on the surface and to increase the surface tension. Polarization, e.g. by flaming, UV treatment, corona treatment or plasma treatment, effects a partial oxidation of the plastic or lacquer surface. Polarization causes hydrophilic groups to form on the plastic and/or lacquer surface and hence makes it more readily wettable by e.g. lacquers.
In the antistatic coating process according to the invention, the surfactant(s) contained in the water can be chosen so as to cause only very little foaming, if any, in the subsequent washing process. Foaming can occur especially when spraying with wash water at high pressures. The foaming, which occurs as a result of the surfactant content and other operating conditions, should not be so intense as to detract from the washing process. It is therefore advisable for at least one of the surfactants in the antistatic solution to be of a non-ionic nature.
In the antistatic coating solution, it is advisable for one or more of the surfactants of the surfactant mixture in the antistatic solution to have a cloud point that is at least 5° C. and at most 40° C. below the temperature of the subsequent washing process used. If surfactants with a cloud point, i.e. surfactants of a non-ionic nature, are used, it is advantageous if, above the cloud point, these surfactants are no longer present in dissolved form in the wash medium of the washing process, so as to minimize foaming, especially when spraying. It can therefore be advantageous to use only non-ionic surfactants. The subsequent washing process is preferably operated by spraying the wash water in the temperature range from 40 to 80° C. and especially from 50 to 65° C. Even though foaming when dipping is often only slight compared with foaming when spraying, operating conditions can occur under which foaming should be reduced when dipping. The temperatures of the washing process by dipping are preferably in the range from 50 to 90° C. and especially from 65 to 80° C. The cloud point is governed by the choice of surfactant(s).
The surfactant(s) in the antistatic solution preferably have a degree of water solubility or a good water solubility, being at least 0.1 g/l in water at 25° C. and preferably at least 1 g/l. The surfactant(s) should also preferably have a degree of solubility or a good solubility in the organic solvent(s). It is advisable for the surfactant or surfactant mixture in the antistatic solution to be completely soluble in the solvent mixture at 25° C. When, after application of the antistatic solution, the organic solvent has extensively or completely evaporated, the surfactant(s) should advantageously be soluble in the remaining amount of water or solvent mixture and/or should have been distributed directly over the surface as homogeneously as possible.
As e.g. plastic injection mouldings are usually still at an elevated temperature for a few minutes after production, the evaporation of the solvent from the solution containing the antistatic agent may be increased. The desired homogeneous surfactant-containing film can thus be formed more rapidly. If, however, the temperature of the mouldings to be coated antistatically is too high, e.g. above the boiling point of the solvent or solvent mixture, the surfactant-containing films may no longer be formed homogeneously.
In terms of chemical composition, the surfactant(s) in the antistatic solution are advantageously similar or identical to the surfactant(s) contained in the aqueous solution used in the washing process. It is therefore advisable for the surfactant(s) in the antistatic solution to be selected from the same chemical group of non-ionic surfactants, especially the groups comprising ethoxylated/propoxylated alkyl alcohols, as the surfactant(s) in the wash medium. With regard to the surfactants used, the surfactant mixture in the antistatic solution can be similar to that in the wash medium.
The surfactant(s) in the antistatic solution and/or in the subsequent washing process should be chosen so as not to have an adverse effect on the subsequent washing process or the properties of the plastic and/or lacquer surface. It is desirable for the surfactant layer remaining on the plastic and/or lacquer surface after washing to consist of only one or a small number of molecular layers.
An unsuitable surfactant can detract not only from the foaming, but also from the wetting, the cleaning quality, the useful life of the wash medium (bath) and hence the mode of action of the washing process. Furthermore, an unsuitable surfactant can cause precipitation due to chemical and/or physical interactions or reactions with other components of the wash medium. Moreover, an unsuitable surfactant in the antistatic solution and/or the washing process may in some cases remain on the plastic and/or lacquer surface in somewhat thicker layers, possibly detracting from the subsequent working steps: For example, during rinsing, which is the last step of the washing process, the rinse baths may become contaminated and surfactant residues may therefore remain on the surface in the subsequent drying process. The surfactant residues accumulating on the plastic and/or lacquer surface can interfere with the lacquer flow (lacquer too thin or absent in places) and the lacquer adhesion, which can impair the external visual appearance of the lacquered surface and the suitability of the lacquer layer due to readily occurring cracking and flaking of the lacquer. Also, an unsuitable surfactant in the antistatic solution can migrate into the plastic or lacquer from the surface, which in particular can create cracks in the plastic moulding or lacquer and affect the mechanical properties of the plastic moulding or lacquer in such a way that the surface region can become more brittle and be damaged more easily by impact, or the strength of the plastic and/or the moulding/lacquer composite can be reduced.
Surprisingly, surfactants or surfactant mixtures have been found which make it possible to avoid all the possible deficiencies in such a process.
The surfactant or surfactant mixture in the antistatic solution is preferably chosen so as to ensure a good wettability of the plastic and/or lacquer surface by the antistatic solution without thereby causing more intense foaming in the subsequent washing process.